Project Summary: microRNAs (miRNAs) are a class of short (21-23bp) double-stranded RNA molecules that silence target mRNAs by translational repression or mRNA destruction. In Drosophila and C. elegans, miRNAs regulate life span. The goal of this project is to understand how molecular defects in one specific Drosophila miRNA, miR-14, cause flies to die young. To this end, the project proposes to develop new biochemical tools to identify the mRNAs regulated by individual miRNA species. These tools will then be used to identify the mRNAs directly regulated by miR-14, in order to determine which miR-14 regulatory targets, when unregulated, are responsible for lifespan reduction. The study should help explain how loss of miR-14 causes accelerated aging in flies, findings that may inform our understanding of aging in mammals by providing candidate mRNAs whose repression could extend life span. The proposal also seeks to discover the general principles by which miRNAs silence gene. Such principles may ultimately inform the design of siRNA as triggers for RNA silencing as a research tool and as a human therapy for disease. Relevance: 'microRNAs,'a class of tiny bits of RNA that turn off other genes, influence aging and other diseases that broadly affect the US and global population, including cancer, Hepatitis C, and HIV/ AIDS. By developing and applying technology to identify which genes a microRNA turns off, this project seeks to understand how a specific microRNA, named miR-14, increases life span in flies. If successful, this project will give insight into how loss of miR-14 shortens life span, how microRNAs chose the genes they turn off, and provide new methods to study which genes are turned off by each of the thousands of individual microRNA species in animals.